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A method has been developed for producing grain refinement in lightweight high manganese and aluminum steels. Many researchers have recently reported that cerium addition produces cerium oxides that serve as heterogeneous nucleation sites for austenite during solidification of Cr-Ni steels. Until now, the use of cerium as a grain refining agent during casting of fully austenitic Fe-Mn-Al-C steels has not been evaluated. In the following study, a Ce-based inoculant in levels of 0, 0.05, and 0.1 wt% Ce was added during the tap and also in-mold to determine the effect of an increasing amount of Ce on reduction in the primary austenite grain size in a Fe-30.5 % Mn-7.55 % Al-0.76 % Si-1.11 % C-0.51 % Mo steel. Addition of 0.05 % Ce during the tap produced only a slight reduction in the grain size. However, in-mold inoculation with 0.1 % Ce produced an order of magnitude reduction in the grain size when compared to the base heat without Ce addition, from 10 to 12 mm to an average of 100 lm. Cerium is very reactive in the melt and readily combines with sulfur and oxygen as well as any residual phosphorus. Inclusion analysis showed that the inclusions in the 0.05 % Ce steel were mainly cerium sulfides while inclusions in the 0.1 % Ce steel were mainly complex cerium oxides, sulfides, and phosphides. Adding cerium greatly increased the nonmetallic inclusion density from 183 to 571 inclusions/mm2 and generally decreased the solution-treated notch toughness from 295 to 141 J. In the as-cast condition and for steels aged for 10 h at 530°C, toughness slightly increased with 0.05 % Ce addition and this is likely the result of the cerium removal of phosphorus from the melt during solidification. Toughness decreased with increasing volume fraction of inclusions and decreasing inclusion spacing regardless of the grain refining effect. It is therefore essential to eliminate sulfur and phosphorus from the melt as much as possible so that cerium will not form sulfides and complex phosphides instead of effective oxide nuclei. This may be achieved with more vigorous calcium addition for desulfurization prior to inoculation with cerium and with the use of nonphosphate bonded refractories to eliminate phosphorus pickup.